1. Technical Field
The present invention relates to a method and apparatus for use in the purification of polluted liquids and other effluents. In particular, the present invention relates to bacterial support media and a method for use of such media in the biological treatment of wastewater.
2. Background Information
Wastewater treatment systems must be capable of converting a stream of wastewater into a condition suitable for discharge into the surrounding ecosystem. Some types of systems contain rotating biological contactors (RBCs), which are used to host aerobic bacteria. The RBCs work to provide those bacteria with a continual supply of all life-supporting ingredients, including food, oxygen and protective housing.
Original RBCs consisted of plastic sheets attached to a central shaft. Bacteria would attach themselves onto the plastic sheets, and the bacteria would be exposed to both food and water by rotating the shaft and sheets into a bath of wastewater. As the bacteria are rotated into the bath, they are exposed to their source of food—the organic matter in the wastewater. As the rotation cycle continues out of the bath, the bacteria receive oxygen. One limitation with this design is that the bacteria are allowed to grow and die in a normal life cycle without a cleansing function. The result is a buildup of dead bacteria carcasses (slough) around the shaft, creating a “dead zone” in the system and a weakening of the shaft.
Subsequent RBC designs attempted to eliminate the buildup of slough by replacing the central shaft and plastic sheets that were used in prior designs with a randomly organized group of plastic media in a rotating cage or basket. One example of such an assembly is described in U.S. Pat. No. 5,401,398, to McManus, which is incorporated herein by reference. The various arrangements of the plastic media provide the surface area onto which the bacteria can attach. The rotation of the basket forces the media into and out of the bath of wastewater, exposing them to the requisite food and oxygen.
One type of media used in these subsequent RBCs, for example the media disclosed in U.S. Pat. No. 5,401,398, to McManus, has a cup-like or hemispherical shape to assist in bailing the wastewater. These shapes, by their basic design, incorporate a “dead zone” at the bottom of the cups where the slough builds up, substantially reducing overall performance of the wastewater treatment system.
Another type of media is a Rashig ring type media, which is described in U.S. Pat. No. 3,540,589, to Boris, in an apparatus for the purification of polluted water, for example. Rashig rings may be short tubular elements that do not contain any protrusions into the interior of the tube. Although this type of media does not have a closed end in which bacteria slough can accumulate, it has the disadvantage that there is insufficient surface area upon which bacteria can attach. Further, this type of media has no mechanism for assuring that the rings undergo sufficient tumbling action to provide maximum contact with the wastewater.
Other types of media have been disclosed by others. For example, U.S. Pat. No. 3,914,351, to McKeown et al., describes a polypropylene media element for use in a packed bed tower where the media has “the form of a truncated cone, the longitudinal axis of which preferably defines an angle of not more than 30° at the hypothetical apex thereof.” The ratio of the diameter to the width (i.e., longitudinal length) is preferably more than 1.5:1. The media may have a series of full length internal ribs. Despite the extra surface area of the ribs, the short stature of the media relative to their diameter make them undesirable for use in rotating digesters, which require media more susceptible to random tumbling rather than an ordered packed arrangement of a packed bed.
Therefore, there is a need for a type of media that may be used in wastewater treatment systems and that overcomes the disadvantages of the prior art by providing sufficient contact area onto which bacteria can attach, reducing the accumulation of slough on the media, and providing sufficient tumbling action to assure maximum efficiency of the wastewater treatment system.